"Open access refers to works that are created with no expectation of direct monetary return and made available at no cost to the reader on the public Internet for purposes of education and research"
(Association of Research Libraries).
Platinum (or sponsored) open access
Making the content of a journal openly available on the publisher's website without any subscription fees or article processing charges. Publishing costs are covered by one or more sponsoring organizations.
Gold open access
Making the final version of a manuscript openly available on the publisher's website immediately upon publication. Publishing costs are covered either by authors who pay article processing charges, by authors' institutions or by publishers.
Green open access (or self-archiving)
Making a version of a manuscript openly available in an institutional repository or an open access disciplinary archive. See the definitions of manuscript versions here.
Hybrid open access
Making individual articles, for which an open access publishing fee has been paid, openly available in a traditional subscription-based journal.
Gratis open access
Providing access to works that is free of charge but not free of some copyright and licensing restrictions.
Libre open access
Providing access to works that is free of charge and free of most copyright and licensing restrictions.